End-to-end Design and Realization of an X-band Transmission Analyzer

The X-band frequency range has been designated for critical military and public safety applications such as satellite communications, radar, terrestrial communications and networking, and space communications. It is important to ensure that these signals deliver quality, reliable, and secure communications. This application note describes the design and realization of a complex X-band transmission analyzer for use in real-time material testing.

The purpose of this analyzer is to gather complex-valued X-band transmission coefficients at high update rates of greater than 100,000 measurements per second. This note describes how manufacturing costs were minimized by integrating the many RF components in the device onto a single printed circuit board (PCB), how coupling issues between the RX and TX paths caused by the requirement for high dynamic range were addressed, and how EM simulator based tuning was used for the numerous distributed elements on the board to ensure optimal performance.

THE DESIGN FLOW

The design team at the Vienna University of Technology was tasked with designing from scratch and realizing this X-band transmission analyzer . The design ﬂow involved the design and optimization of several breeds of circuits, including critical elements such as bias-T and microstrip ﬁlters, all of which were designed using AWR’s circuit, system, and EM analysis software within the single, integrated AWR Design Environment™.

The PCB layout was done entirely within AWR’s Microwave Ofﬁce® circuit design software. Additionally, AWR’s Visual System Simulator™ (VSS) communication system design software was used to ﬁnd an optimal RX chain and to estimate the phase locked loop’s (PLL) phase noise properties. The PLL’s loop ﬁlter characteristics were optimized using the exact same schematic that was also used for carrying out the PCB layout. Finally, AWR’s AXIEM® 3D planar EM software was utilized for simulating the varied distributed element circuits, as well as to tackle shielding issues in the ﬁnal PCB design.(Figure 1 shows all relevant RF-circuits that were investigated during this project in order to design the resulting ﬁnal, outstanding overall system.)

Figure 1. PCB topologies investigated during the design flow.

Because all AWR’s technologies are integrated into a single design environment, the team was able to easily reuse structures and circuits from system models down to PCB layout structures. For example, EM simulations of the actual PCB traces could be checked against VSS models to see whether the designed shielding was sufﬁ cient. This approach enabled the designers to reuse highly optimized sub circuits in the ﬁ nal PCB design (Figures 2 and 3).

Microwave Ofﬁce software enables users to readily tune circuits based upon EM simulations. In this case, the design required that the X-band RF link, the high-speed serial bus, and the DC power supply be combined onto the same cable, thus requiring a bias-T for each and every X-band cable interface.

Figure 3. Final manufactured prototype.

In order to reduce assembly complexity and manufacturing costs, the circuit was realized using distributed elements. A classic radial stub approach was determined to be the best, considering tradeoffs between PCB real estate, circuit performance, and board complexity. For reasons emerging from signal post-processing, two different-sized radial stubs were used to ensure that the circuit would perform consistently over about 30 percent bandwidth (Figure 4). Using two unequal stubs made it possible to achieve constant low RF leakage over a broad frequency range, however, this architecture was more challenging to design because placing resonant structures in close proximity resulted in significant coupling effects. Typically this is less of a problem when dealing with multiple structures, all operating at the same frequency. Yet in this case, the coupling effects tended to disrupt the phase relations if not all length ratios were close to optimum.

Figure 4. Bias-T dimensions.

It was clear to the team that this issue could not be resolved using closed form approximations or simple models. For this reason, AXIEM and the Microwave Ofﬁce optimizers were the weapons of choice. Numerous runs were necessary to ﬁ nd a proper solution and to answer questions like “what’s the required clearance to the surrounding ground?” In the end, the same circuit was reused several times within the overall design without any issues.

The inner workings of the circuit could be understood using the current density derived by AXIEM (Figure 5).

Figure 5. Bias-T standing wave current density.

The current density’s maximum, which is equal to the phase center of the RF reﬂ ect, shifted with respect to the excitation frequency. This resulted in a frequency-independent virtual RF open exactly at the branch line intersection with the RF path. This was observed as current density minimum at this position. Therefore, the branch line was virtually “invisible” for the RF signal. All the RF leakage measurements as well as the simulations were combined, as shown in Figure 6. The ﬁnal circuit indeed shows the desired ﬂ at frequency response in leakage over a broad band and at constant low leakage.

Figure 6. Measured data vs. simulation.

THE MICROSTRIP FILTER

The microstrip ﬁlter design was carried out by deriving an equivalent lumped circuit from the design’s speciﬁ cations. This representation can easily be reformulated in terms of characteristic impedances, which is the starting point for any distributed ﬁlter design. After that, it is up to the designer to choose a transmission line topology that 1) can realize all the desired impedances, 2) enables a compact setup, and 3) can be manufactured with an available PCB process. In this situation, the Microwave Ofﬁce software environment assisted the design team with various tools like TX-Line® and simple transmission line models available for many topologies. It further enabled the designers to quickly gauge whether a certain substrate, topology, and tolerance mix could work. These decisions are critical to the overall success of the design and must be carefully considered.

Figure 7. X-band filters (standing wave current density).

The challenge with designing the microstrip ﬁ lter for this project was that the substrate needed to be quite thin in order to achieve a compact design in a 50Ω microstrip stack up. It was also problematic for the PCB manufacturer to deal with very narrow coupling gaps. The design team needed to ﬁnd an alternative to the classic edge-coupled microstrip design. The resultant design utilized two lines with higher characteristic impedance in parallel instead of a single line, which resulted in a larger range of achievable characteristic impedances. Figure 7 shows the two ﬁ lters in stop-band and pass-band excitation. Even though the design was complex due to the extensive use of circuit parameters, the simulation power as well as the hierarchical design capabilities of the Microwave Ofﬁ ce software made it possible to run the same optimization routines on both circuits. This not only reduced the design team’s time and effort, but also meant that the two blocks were exchangeable in the ﬁnal design.

Figure 8a. final prototype of the X-band transmission analyzer (outside).

CONCLUSION

This application note illustrates a complete design flow for the end-to-end design and realization of an X-band transmission analyzer. The ability to not only design and optimize several different circuits on a single PCB but also to work through many design iterations and verification steps at different abstraction layers was critical to achieving the project’s ambitious performance goals. Keeping design changes and parameter variations consistent through all abstraction layers was a challenging task that was made possible and ultimately successful with the help of the AWR Design Environment that offered complete integration of all the constituent design and verification steps.

Figure 8b. Final prototype of the X0band transmission analyzer (inside).